1 Field of the Invention
The present invention relates to conductivity anomaly detection systems, and more particularly to a superconducting conductivity anomaly detection system using a rotating superconducting magnet as a transmitter and a plurality of superconducting quantum interference field detectors (SQUIDS) having their detection coils rotating with the superconducting magnet as a receiver to detect the second harmonic of the rotation frequency.
2. Description of the Prior Art
A conductivity anomaly detection (CAD) system detects anomalies produced in the time varying magnetic field of a controllable source, which anomalies are created by a target of high conductivity. In the case of submarines the highly conducting target is itself in a background, the ocean, which has moderate conductivity. A CAD system has advantages such as (a) less susceptibility to external noise sources since it operates in a very low frequency range where there is less noise, (b) the ability to measure depth of the target since the phase of the return signal is a function of depth below the surface, and (c) the capability of localizing a target since the CAD frequency is fixed. Such a CAD system is limited by the strength of the magnets which generate the known source field which in turn requires a detector of extreme sensitivity, i.e. a superconducting magnetometer (SQUID).
The CAD system is an "active" search system which means a controllable signal source is used to determine the location of a target. The operational frequency is very low so the detector must measure slight changes in impedance rather than a reflected electromagnetic wave. Alternating fields are set up by a strong source magnet which is driven at a fixed frequency. When a conducting target is present, the alternating fields around it induce currents which in turn produce weak new fields. The weak new fields are detected as a small anomaly in the field pattern of the source.
Many airborne electromagnetic prospecting and detection systems have been devised and employed for years to detect conducting mineral deposits in the earth, a background which has virtually zero conductivity. Some major problems common to all such systems include: how to generate large amounts of transmitted power to achieve long range and depth penetration; how to separate a small target signal from this large transmitted power; and how to separate the small target signal from other large signals that also originate in the environment.
Rotating magnetic fields have been generated with two orthogonal coils fixed to an aircraft and driven in phase quadrature. Detection was accomplished by coils in a towed bird some distance behind and below the aircraft. This system, first tested over thirty years ago, is of negligible value due to the irregular motion of the detection coils in the earth's magnetic field and the undesired coupling between the transmitting and detecting coils.